There is an electronic mail transmission/reception system composed of a mail server and a plurality of client terminal devices connected to the mail server, in which the mail server stores the mail bodies of electronic mail transmitted from the client terminal devices, independently of attachments, and when the mail server is requested by any client terminal device to transfer an attachment, the mail server transfers the attachment to the client terminal device (see Patent Document 1). The mail server counts the number of requests for an attachment per predetermined period of time based on requests by the client terminal devices to transfer the attachment, and when the counted number of requests is equal to or greater than a predetermined value, generates and stores replicated data for the attachment. Upon reception of a request by any client terminal device to transfer the attachment, the mail server transfers the attachment or the replicated data to the client terminal device. In this electronic mail transmission/reception system, the mail server generates replicated data for attachments only when attachment transfer requests are concentrated within a short period of time, and therefore the mail server's memory space does not decrease significantly, so that concentration of attachment transfer requests can be avoided.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-333099